The intermittent energization of such load-operating impedance elements may be controlled by diverse command means responsive to one or more variables such as temperature and/or time. U.S. Pat. No. 3,948,441 to Perkins et al., for example, describes a two-wire circuit including a thermostatic switch whose set point is periodically changed by a timing mechanism in order to maintain different mean room temperatures by day and at night. Such a time-dependent variation in the operating point of a switch responsive to an external parameter could also be based on seasonal rather than diurnal changes, if desired.
In a system of this nature it is of course necessary that the thermostatic or other command means (possibly including a timer) remain operational even during OFF periods of the controlled load impedance. Thus, the above-identified Perkins et al. patent teaches the provision of a storage battery which is charged from the supply when the thermostatic switch is open and which supplies the timing mechanism with the necessary operating energy.